1. Field of the Invention
The invention relates to a broadband balancing transformer—balun—for transferring large high frequency powers from an unbalanced connection to a balanced bipolar connection and vice versa.
2. Description of the Related Art
For high frequency applications inter alia in the higher power range, for example in amplifiers, which are constructed to produce the sought power level from power transistors in push-pull technology, balancing transformers are necessary. These convert the high frequency signal from an unbalanced coaxial line or strip line into two signal lines which are designed to be as balanced as possible relative to each other in order to supply it to two power transistors or push-pull transistor operating in a balanced manner. Analogously, the two balanced output signals of the two power transistors or of the push-pull transistor can be converted via a balancing transformer into a high frequency signal for an unbalanced coaxial line or strip line.
In particular, production of the balancing transformer by means of strip conductors which are disposed on a printed circuit board in strip line technology is hereby desired since, with this type of production, in comparison to coaxial conductor technology, the technical manufacturing reproducibility of the balancing transformer including its electrical properties is ensured in mass production. In addition to good technical manufacturing reproducibility of the balancing transformer, strip line technology, relative to coaxial line technology, is characterized by a smaller constructional volume and lower manufacturing costs.
In EP 0 418 538 A1, a balancing transformer of this type is represented. The transfer of the high frequency signal power is hereby effected by transformational coupling between two conductor loops, one of which is connected to the unipolar connection of the unbalanced line and the other conductor loop to the bipolar balanced connection to the two power transistor amplifiers. A good transformational coupling between the two conductor loops is achieved in that the two conductor loops are produced with respect to their geometric position as strip conductors which are aligned relative to each other on the upper or lower side of a printed circuit board.
Balanced transfer ratios on the side of the balanced transformer orientated towards the power transistor amplifier are produced via the electromagnetic coupling. For this purpose, the two balanced connections of the balancing transformer are led to earth according to FIG. 1 on the side of the power transistor amplifier via symmetrically dimensioned conductor loop regions. With respect to a balanced conversion within the balancing transformer, the conductor loop on the unbalanced side which is led to earth via symmetrically dimensioned conductor loop regions is disposed in such a manner on the printed circuit board that a balanced transformational coupling is produced between these two balanced conductor loop regions on the side of the unbalanced line and the two balanced conductor loop regions on the side of the power transistor amplifier. In this way, a balanced power distribution from the unbalanced line to the two balanced poles on the side of the amplifier is ensured.
A more precise analysis of the transfer behaviour of the balancing transformer of EP 0 418 538 A1 by means of a field analyzer, such as illustrated in FIG. 2, reveals that, with a given design of the balancing transformer on the basis of clearly pronounced resonances in the transfer behaviour at approx. 1.8 GHz, a meaningful operation of the balancing transformer can be recommended only up to a frequency range of approx. 860 MHz. In addition, with a given design of the balancing transformer itself in this narrow band frequency range up to 860 MHz, the transfer behaviour at the signal input on the unbalanced side is only conditionally balanced relative to the two outputs on the amplifier side of the balancing transformer (S12, S13).